During the past decades there have been considerable developments within the field of radiation therapy. The performance of external beam radiation therapy accelerators, brachytherapy and other specialized radiation therapy equipment has improved rapidly. Developments taking place in the quality and adaptability of radiation beams have included new targets and filters, improved accelerators, increased flexibility in beam-shaping through new applicators, collimator and scanning systems and beam compensation techniques, and improved dosimetric and geometric treatment verification methods have been introduced.
Furthermore, a number of powerful 3-dimensional diagnostic techniques have been developed, ranging from computed tomography (CT), positron and single photon emission computed tomography (PET and SPECT) to ultrasound and magnetic resonance imaging and spectroscopy (MRI and MRS). Equally important is the increased knowledge of the biological effect of fractionated uniform and non-uniform dose delivery to tumors and normal tissues and new assay techniques, including the determination of effective cell doubling times and individual tissue sensitives, allowing optimization of the dose delivery to tumors of complex shape and advanced stages.
However, one of the weakest links in this development in radiation or radiation therapy treatment has been delivering correct dose to the target volume, including tumor tissues, in a patient. In order to guarantee accurate dose delivery, detailed anatomical information of the tumor, surrounding tissues, organs and bone structures is required. From this information, the target volume with the tumor is defined in the patient body in relation to some reference points or structures, e.g. adjacent bones or standard anatomical reference points used in radiation therapy. During the treatment in a radiation therapy machine, the target volume is then aligned relative the treatment radiation source based on these associated reference points. In other words, an indirect alignment of the target volume is performed, since the position of the reference points and not the actual target volume is used. However, the target volume with the tumor is a fluid structure and its position relative the reference points is not rigid, but may change depending on e.g. posture of the patient, filling degree of bladder, respiratory motion, etc. Therefore, although the reference points are aligned correctly in relation to the treatment radiation source, the target volume may be misaligned.
In addition, during the treatment procedure, the spatial relationship between the target volume and the reference points and the shape and size of the tumor may change, due to loss of weight, changes in the filling degree of bladder and changes in tumor size caused by the already delivered radiation doses. Thus, the received dose in the target volume in a subsequent treatment occasion may differ from an ideal or expected dose. In some extremes, the radiation dose may actually partly or completely miss the target volume and instead hit adjacent tissues and organs. This not only makes the treatment ineffective, but may also harm healthy tissue in the patient.
Today, diagnostic imaging machines have to be used between different treatment occasions to evaluate the dose delivery and detect changes in tumor size and position. However, this is an ineffective and expensive solution, since the patient then has to be moved between different machines, i.e. the diagnostic machines and the treatment machine. In addition, the position and posture of the patient in the machines, most often, are not identical and therefore the position of the tumor relative the reference points differs between the machines.
A method for aligning a patient for radiation treatment in a radiation therapy machine incorporating a computed tomography functioning is shown in U.S. Pat. No. 5,673,300. In a gantry of the radiation therapy machine, an X-ray source collimated to produce a fan beam and an associated detector are arranged to produce tomographic scans of a patient. The gantry also comprises a treatment radiation source emitting a fan beam of high-energy radiation to a target volume in the patient and a dedicated detector adapted to receive the high-energy beam passing through the patient. From an earlier tomographic patient scan, projection images are used to reconstruct a tomographic image. These images are then compared to projection images taken at the time of the radiation therapy to determine a series of offsets of the patient, which may be used to characterize and correct for motion of the patient between the initial tomographic scan, used for treatment planning, and one or a series of subsequent radiation treatment sessions.
The radiation therapy machine in U.S. Pat. No. 5,673,300 divides and irradiates the target volume in a plurality of slices. If the patient moves slightly during the actual dose delivery, a major portion or the whole radiation dose will miss the actual slice. Thus, an incorrect and inefficient radiation delivery is accomplished, possibly irradiating sensitive tissues and organs and causing more harm than good. In addition, due to imperfections of the collimation, scattering of the fan beam causes some radiation to be delivered to the patient out of the intended actual slice. Therefore, the irradiated slices receive too low radiation doses, whereas surrounding tissues and organs receives a too high dose.
In the international application WO 01/60236, a radiation therapy system is disclosed. The system includes a radiation source that moves about a path and directs a beam of radiation towards an object and a cone-beam computer tomography (CT) system. The cone-beam CT system includes an X-ray source that emits an X-ray beam in a cone-beam form towards an object to be imaged and an amorphous silicon flat-panel imager receiving X-rays after passing through the object, the imager providing an image of the object. A computer is connected to the radiation source and the cone-beam CT system, wherein the computer receives the image of the object and based on the image sends a signal to the radiation source that controls the path of the radiation source.
The general L or C shaped gantry of the radiation therapy system in WO 01/60236 is designed for a rotation speed according to recommendations of the International Electromechanical Commission (IEC), i.e. about 1 minute per revolution. The rotational support of the system is provided at one axial end of the body, which together with the heavy weights may cause the gantry arms to elastically deform, especially for a rotation speed faster than 1 minute per revolution. Thus, instead of a pure rotation, the gantry will precess, creating an inaccuracy in the positioning of the radiation head. However, if the cone-beam CT system is to function efficiently much faster rotation speeds than 1 minute per revolution are required. During the 1 minute of revolution, the patient may move considerably, whereby an inaccurate and misleading CT image is obtained.